This invention generally relates to diet supplements for aquaculture species, especially catfish, for increasing the rate and efficiency of fish growth.
A major function of lipids in modern nutrition is to serve as a substrate for production of metabolic energy. Mechanisms regulating the production of metabolic energy under a wide variety of physiological conditions are required for survival of the species. The critical role of carnitine in the production of energy from long-chain fatty acids is well recognized. Carnitine also has a role in the production of metabolic energy from several substrates in addition to long-chain fatty acids. As a result, adequate carnitine is essential in maintaining health.
Unlike most vitamins and vitamin-like substances, carnitine was identified and synthesized long before the discovery of its nutritional role. Carnitine was first found in muscle extracts by two Russian scientists in 1905, identified as beta-hydroxy-alpha-butyrobetaine, and named from the latin carnis, meaning flesh or meat. In the late 1940's, Fraenkel discovered that carnitine was a required substrate for the mealworm Tenebrio molitor. He named it vitamin B.sub.r, although it was later established that carnitine is not a vitamin for higher organisms since most of the animal requirement is fulfilled by biosynthesis. Early research literature also calls carnitine vitamin B.sub.11. In 1959, Fritz found that carnitine stimulated the rate of fat burning (called "beta-oxidation"). Subsequent investigations revealed that the mechanism of carnitine action was the transport of fats by a carnitine-dependent mechanism into the mitochondria where they are utilized for energy.
Carnitine is chemically termed 3-hydroxy-4-N-trimethylamino butyric acid; it is similar to choline and synthesized in animals from amino acids. However, unlike amino acids, carnitine is not used for protein synthesis. Carnitine, like many other biological molecules, comes in two forms: L-carnitine and D-carnitine.
Although these isomers are mirror images of each other, only the L-isomer is biologically active. The D-form is completely inactive, and may even inhibit the utilization of L-carnitine. Whether supplied by the diet or from endogenous synthesis, carnitine is essential in the metabolism and movement of fatty acids within and between cells. An enzyme, carnitine acyltransferase, has been found to be part of the mechanism for releasing CoA and acyl-CoA. The effect of carnitine on fatty acid metabolism seems to be limited to fatty acids with chain lengths greater than C.sub.8. Since palmitylcarnitine also stimulates fat synthesis in livers, another vitamin role of carnitine may be in the regulation of lipogenesis.
Most organisms have the ability to produce their own carnitine. The endogenous production of carnitine appears to occur mainly in the liver, and requires two amino acids, lysine and methionine, three vitamins, vitamin B.sub.3 (niacin), vitamin B.sub.6 and vitamin C (ascorbic acid), and iron. Trimethyl-lysine is produced by methylation of lysine using a methyl group from methionine. The trimethyl-lysine is converted to an aldehyde using PALP as a co-factor, which is oxidized to a butyrate by an NAD-linked dehydrogenase. The butyrate is then hydroxylated by a ketoglutarate-ferrous ascorbate compound to form carnitine.
The role of carnitine in nutrition received little attention until 1973, when the first carnitine-deficient human patient was described. Since then, many clinical investigations have focused on biomedical aspects of carnitine deficiency, as well as on the effects of supplementary dietary carnitine on disease processes. No deficiency problems in normal vertebrates have yet been found under practical conditions. Nevertheless, young rats, chick embryos and rabbits on a low level of nutrition have all been shown to grow more rapidly when carnitine has been supplied directly or indirectly. There have also been reports that supplementation of an adequate diet of young pigs with carnitine may enhance growth.
One important, and as yet unresolved, issue is the relative contribution of diet and biosynthesis to the total carnitine intake. Some animal work, particularly studies conducted on mammals, has been published in this area indicating that biosynthesis of carnitine in adult animals is far more important than diet.
Improving feed efficiency and feed responses is particularly desirable in view of the rising cost of the commodities used to prepare the feeds. Commercial aquaculture, especially of catfish and crustaceans, has grown rapidly over the last several years, yet few improvements in feed composition or efficiency have been made. Channel catfish, for example, are commonly fed diets with 32 to 36% crude protein, with most of the protein in the form of soybean meal and menhaden fish meal. In recent years, fish meal has sometimes been replaced with a combination of soybean meal and meat-bone meal. The increase in plant protein concentrate may cause the diet to be less digestible and perhaps less metabolizable. Fish fed diets of plant material and higher amounts of carbohydrate or fat may develop lipid accumulation in certain tissues, such as the liver and muscle, rather than convert the energy into growth. Garling, D. L. Jr. et al., "Effects of Dietary Carbohydrate-to-Lipid Ratios on Growth and Body Composition of Fingerling Channel Catfish" Prog. Fish-Cult. 39(1), 43-47 (1977).
Researchers have investigated several additives for their effect on growth and response to stress in fish. Most of these additives are purified or synthesized forms of vitamins, essential amino acids, and essential fatty acids. Others are digestive aids which include individual enzymes and mixtures of gastric enzymes. Growth or sex hormones have been added to culture water or have been dissolved in acetone prior to coating onto the surface of fish food. In these cases, extra additive is used to compensate for losses from the leaching or diluting effect of water. In one example, in Italy, administration of L-carnitine to the culture water was credited for increasing growth of sea bass fry. Santulli, A. et al., "The Effects of Carnitine on Growth of Sea Bass", J. Fish Biol. 28, 81-86 (1986). Santulli, A. et al., in "Supplemental Dietary Carnitine Effects on Growth and Lipid Metabolism of Hatchery-reared Sea Bass (Dicentrarchus labrax L.)", Aquaculture 59, 177-86 (1986), demonstrated that carnitine containing diets, prepared using a process normally used to incorporate fat soluble hormones rather than water soluble materials, increased specific growth rate and reduced liver and muscle lipid concentrations of sea bass. The diets were prepared by soaking the feed composition in a carnitine solution and drying, to a final concentration of approximately 2.0% L-carnitine by weight of dry feed. The actual amount fed to the fish cannot be ascertained based on the information provided but was less than the 2.0% because of leaching of substantial amounts of the water soluble carnitine into the water.
It is therefore an object of the present invention to provide commercial fish diets containing low doses of L-carnitine which are effective in increasing the rates of weight gain and feed efficiency.
It is a further object of the present invention to provide commercial fish diets which reduce fat in processed fish.
It is a still further object of the present invention to provide commercial fish diets which can be used to increase resistance to stress.
It is another object of the present invention to provide commercial warmwater fish diets which specifically increase resistance to ammonia toxicity.